A large number and variety of tests for different diagnostic purposes directed to body fluids such as saliva, urine, blood, cerebrospinal fluid, ascites, etc., have been developed. This invention is directed primarily to diagnostic tests applied to blood, which may for example be obtained from a drop or two squeezed from a finger prick, or may be blood drawn into an anti-coagulant for test purposes or for transfusion, or any other blood product in which red cells are suspended in plasma or in saline, or in other fluid compatible with red cells. Many of these tests rely on colorimetric or spectrophotometric evaluation of a reaction of a fluid component with one or more specific reagents. Such diagnostic evaluation, primarily for different anomalies, has been the focus of increasingly refined clinical techniques and devices.
A particularly important and very frequently tested body fluid is blood plasma, most often obtained in hospital practice by venipuncture followed by withdrawal of 5 cc or more of blood, which in turn is spun in a centrifuge. The red cells settle to the tube bottom, leaving the clear plasma in the upper portion. This procedure entails the usual hazards of venipuncture, and is not generally feasible in other than hospital environments. Many test procedures require as little as 5 microliters (.mu.L), or about 1/5th to 1/10th of the volume of a drop of blood. The subject invention is addressed to methods and devices which provide convenient and economical means for obtaining about 3 to 30 .mu.L of plasma, and to combinations thereof with the other components of diagnostic devices.
It is important in such tests to assure an absence of substances interfering with the target analyte. It is equally important that the plasma be obtained with all of its components present, and with an undetectable level of removal during the separation. For example, none of the factors I through XII (which constitute a cascade essential to the clotting of blood) should be significantly diminished from their original concentration in the blood plasma. If the separation is accomplished by passage through fine fibrous media, a number of these and of other factors, such as hormones, proteins, nucleic acids, enzymes, growth factors, lipoproteins, and other important normal and abnormal blood components such as heparin tend to be removed by adsorption to the fiber surfaces.
Further, it is highly desirable that components of the immune system, including the numerous proteins which constitute complement, be neither removed nor activated by contact with the fiber surfaces.
Finally, hemolysis of the red cells must be avoided, as the intense red color of released hemoglobin would interfere with diagnostic tests which employ color change as a part of their procedure.
The devices and methods in accordance with this invention make it possible to obtain the quantities required for these tests from one or two drops of blood, which is readily and rapidly obtainable, for example in a doctor's office, or by an individual for himself. By using the method of this invention, the specimen so obtained is identical with or differs minimally in composition with that obtained by centrifuging.